Group A Streptococcus (GAS) is an important human pathogen affecting millions people globally each year. In addition to the direct infectious morbidity, numerous patients also suffer post-infectious sequelae including rheumatic fever, a major cause of valvular heart disease throughout the world. Streptokinase (SK) is a major GAS virulence factor and the streptokinase/plasminogen interaction has been established as a critical factor in streptococcal pathogenesis. However, the precise mechanism underlying the pathogenicity of SK in GAS infection is still unclear. In this proposal, it is hypothesized that local thrombosis plays a critical role in host defense against bacterial infection and that GAS utilizes SK to hijack the host fibrinolytic system to bypass this host defense and facilitate bacterial infection. This model predicts that genetic variations among hemostatic factors might affect host susceptibility to GAS infection. The studies outlined in this proposal will study various aspects of SK's roles in GAS pathogenicity. In Specific Aim 1 the impact of novel SK inhibitors on GAS pathogenicity will be tested and the role of SK in GAS invasiveness will be studied by comparing the systemic spread of wild type and SK knockout GAS strains in mice expressing human plasminogen. The temporal requirement of SK in GAS infection will be determined. In addition, the roles of fibrin-dependent and fibrin-independent SK activation of host plasminogen in GAS pathogenicity will be explored by comparing the virulence of GAS expressing wild type SK versus SK60-414, a fibrin-dependent mutant. In Specific Aim 2 the effects of variations in host hemostatic factors, such as Factor V and fibrinogen, on host susceptibility to GAS infection will be tested. The relative contributions of plasma and platelet FV pools to host defense will be studied. Fibrinogen null mice will be used to study the role of fibrinogen in initiation of GAS infection. The role of fibrinogen and GAS M protein in GAS phagocytosis resistance will be also investigated. Data collected from the above studies will further our understanding of the contribution of host hemostatic system to host susceptibility to bacterial infection. As a result, alternative approach to treat bacterial infection by interfering with host hemostatic system's interaction with bacterial can be explored in addition to antibiotic treatment. Group A Streptococcus (GAS) is an important human pathogen that causes millions cases of infection as well as post infectious diseases. Host hemostatic system has been shown to be involved in GAS infection in our previous studies. The overall objective of this proposal is to understand how the host hemostatic system interacts with GAS and to identify novel therapeutic targets for treatment of GAS infections.